Structure and rheology correlation in polymer nanocomposites
Considerable efforts have recently been devoted to the study of a new class of filled polymers, nanocomposites, in which very small particles are dispersed in a polymer matrix. Polymer-layered silicate (PLS) nanocomposites are a well-known example, and special attention has been paid to these materials due to their potential for enhanced properties, such as thermal and dimensional stability, lower permeability, better surface finishing and printability, improved biodegradability, and to some extent enhanced mechanical behavior.1–5Most of these properties are obtained through partial or total exfoliation of the lamellae and their homogeneous distribution within the polymer matrix. These conditions can be created in a solvent medium or in molten state using mixing processes that are governed by the flow pattern and rheology of the system.6 Accordingly, quantitative understanding of the rheological properties of such nanocomposites is important for mastering the process parameters that lead to controlled microstructure (exfoliation and distribution of the lamellae), which in turn enables control of the end-use properties.7–10
Among the various rheological tests, transient experiments such as stress growth after step-up in shear rate are known to be very sensitive to the microstructure of complex fluids such as polymer multiphase systems.11–13 In the case of PLS nanocomposites, the orientation and organization of silicate layers under flow, and to some extent the creation and destruction of the clay network structure, can be evaluated by analyzing the location and the amplitude of the stress overshoot during the start-up experiment.7–11,14,15 To investigate the structure build-up at rest in PLS nanocomposites, flow-reversal experiments are usually conducted at different rest times after cessation of the forward start-up flow. Structure recovery during the rest time can then be determined by measuring the reappearance of a stress overshoot during the reverse
start-up flow. In flow-reversal experiments, the sample is first subjected to a continuous shear flow until a steady state is reached. The flow is then stopped and the sample is kept at rest for a given period of time. Finally, the sample is sheared in the reverse direction until a steady state is reached.
We prepared nanocomposites by melt-mixing polybutylene succinate-co-adipate (PBSA) and Cloisite 30B (C30B) using a thermo Haake mixer. The mean interlayer spacing of C30B and the nanocomposites was determined by x-ray diffraction. The flow-reversal experiments were performed in a strain-controlled rheometer in parallel-plates geometry. After loading the sample and before starting the test, a pre-shear was applied. X-rays in transmission mode were used to measure the orientation of the clay platelets in samples cooled in the rheometer both before the forward start-up test (sample 1) and at the end of the test (sample 2).
Figure 1 shows the x-ray diffraction (XRD) patterns of PBSA, C30B, and a nanocomposite with 7.5%wt C30B. The diffraction peak of the nanocomposite shifts to lower angles, an indication of increasing interlayer spacing in the clay platelets. Figure 2 shows the results of stress growth experiments for forward and reverse flows with different rest times (flow-reversal experiment) and exhibits a clear stress overshoot for the forward flow. However, no significant overshoot is observed for the reverse flow after a short rest time. With increasing rest time, the stress overshoot amplitude increases gradually and approaches that of the forward flow at a relatively long rest time. This can be attributed to the disorientation of silicate layers during the rest period, which may be related to the attractive interactions among platelets that may be boosted by Brownian motion.11
Transmission mode x-ray analysis reveals no evidence of the orientation of clay platelets in sample 1—see Figure 3(a)—which means that they are randomly oriented in the polymer matrix before starting the rheological test. However, when the sample is submitted to shear flow—see Figure 3(b)—the clay platelets are oriented in the flow direction, indicated by an elongated light spot in the x-ray image.11
Our experiments show that the amplitude of the stress overshoot in the reverse flow gradually increases with increasing rest time, and with a relatively long rest time becomes essentially the same as the forward flow. Reappearance of the stress overshoot (i.e., structure recovery in the polymer nanocomposite) is attributed to the loss of orientation of silicate layers during the rest period. The results also show that after subjecting the nanocomposite to shear flow, the initially randomly oriented clay platelets become oriented in the flow direction.
As next steps we plan to build on this work with further experiments and adequate modeling. Our conclusion thus far is that the orientation of the clay lamellae by flow—which has implications for barrier properties, among others—can be lost if the nanocomposite is left at rest for a given period of time or entrapped in areas of the machinery where the shear rate is low. The overall control of the orientation depends on the flow geometry and the specific features of the machinery, the rheological properties of the matrix, the concentration and the aspect ratio of the clay lamellae, and the specific interactions at the interface between the matrix and the clay lamellae, and between the lamellae themselves.
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